The ESP8266WiFi library is written for use when directly programming the ESP8266, which is why you got that error. You have no chance of ever being able to compile it for the Arduino/Genuino Uno. If you actually are directly programming an ESP8266, then you need to select the correct board from the Tools > Board menu. If you are programming an Uno that has an ESP8266 connected to it, then you can't use that library. I recommend this one instead:https://github.com/bportaluri/WiFiEsp

The ESP8266WiFi library is part of the ESP8266 Core For Arduino. It is installed when you install the complete ESP8266 core and the version of the library installed was written specifically to go with the version of the core. There is no good reason to install the library separate and doing so will likely cause you problems eventually. I'm actually quite curious how you and all the other beginners even manage to get hold of the library by itself. It would be a project to get it out of the ESP8266 Core For Arduino. Please delete C:\Program Files (x86)\Arduino\libraries\ESP8266WiFi.

Another thing: you should never install libraries or save anything else to the Arduino IDE installation folder. The reason is that everything you save to that folder will be lost when you update to a new version of the Arduino IDE. You should always install libraries to the libraries subfolder of your sketchbook folder. The Arduino IDE's Sketch > Include Library > Manage Libraries and Sketch > Include Library > Add .ZIP Library will automatically install libraries to the correct location, so you should use those features whenever possible. Please study the official guide to installing libraries to learn the correct way to install libraries:https://www.arduino.cc/en/guide/libraries